The major long-term objective of this study is to examine immune reactivity to the PM-1 antigen and its relationship to the pathogenesis of polymyositis-dermatomyositis. Specific objectives are to: 1. Pursue purification and characterization of the PM-1 antigen. 2. Develop more sensitive tests for PM-1 antigen and antibody. 3. Determine the specificity of the PM-1 antibody for polymyositis and dermatomyositis. 4. Determine the relationship between the presence or absence and level of PM-1 antibody and clinical manifestations in polymyositis and dermatomyositis. 5. Determine the species, organ and cellular specificity of the PM-1 system. 6. Characterize the regulation of immune responses to PM-1 antigen and their relationship to the production and regulation of experimental autoimmune myositis. Methods will include immunodiffusion, hemagglutination, immunofluorescence, affinity chromatography, and electrophoretic analyses of antigens and antibodies obtained from human and experimental animal sources.